Add Husky, Prettier, ESLint, and Vitest hooks

[lowyelling]

Summary

• Add Husky git hooks (pre-commit, commit-msg, pre-push)
• Add lint-staged to run ESLint + Prettier on staged files
• Add Prettier with project config
• Add Vitest for testing
• Add commitlint to enforce conventional commit messages
• Add mermaid diagram skill
• Format existing codebase with Prettier

Test plan

• Pre-commit hook runs lint-staged and vitest
• Commit-msg hook enforces conventional commits
• Pre-push hook runs full build
• Prettier formats all files cleanly

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